To understand autoloading in Zend Framework, first you need to understand the relationship between class names and class files.

Zend Framework has borrowed an idea from » PEAR, whereby class names have a 1:1 relationship with the filesystem. Simply put, the underscore character ("_") is replaced by a directory separator in order to resolve the path to the file, and then the suffix ".php" is added. For example, the class "Foo_Bar_Baz" would correspond to "Foo/Bar/Baz.php" on the filesystem. The assumption is also that the classes may be resolved via PHP's include_path setting, which allows both include() and require() to find the filename via a relative path lookup on the include_path.

Additionally, per PEAR as well as the » PHP project, we use and recommend using a vendor or project prefix for your code. What this means is that all classes you write will share a common class prefix; for example, all code in Zend Framework has the prefix "Zend_". This naming convention helps prevent naming collisions. Within Zend Framework, we often refer to this as the "namespace" prefix; be careful not to confuse it with PHP's native namespace implementation.

Zend Framework follows these simple rules internally, and our coding standards encourage that you do so as well for all library code.

Zend Framework's autoloading support, provided primarily via Zend_Loader_Autoloader, has the following goals and design elements:

• Provide namespace matching. If the class namespace prefix is not in a list of registered namespaces, return FALSE immediately. This allows for more optimistic matching, as well as fallback to other autoloaders.

• Allow the autoloader to act as a fallback autoloader. In the case where a team may be widely distributed, or using an undetermined set of namespace prefixes, the autoloader should still be configurable such that it will attempt to match any namespace prefix. It will be noted, however, that this practice is not recommended, as it can lead to unnecessary lookups.

• Allow toggling error suppression. We feel -- and the greater PHP community does as well -- that error suppression is a bad idea. It's expensive, and it masks very real application problems. So, by default, it should be off. However, if a developer insists that it be on, we allow toggling it on.

• Allow specifying custom callbacks for autoloading. Some developers don't want to use Zend_Loader::loadClass() for autoloading, but still want to make use of Zend Framework's mechanisms. Zend_Loader_Autoloader allows specyfing an alternate callback for autoloading.

• Allow manipulation of the SPL autoload callback chain. The purpose of this is to allow specifying additional autoloaders -- for instance, resource loaders for classes that don't have a 1:1 mapping to the filesystem -- to be registered before or after the primary Zend Framework autoloader.